1. Field of the Invention
This invention relates to a vacuum-forming system for use in gas sorption analyses used to measure surface area and pore analysis of materials, and specifically to an improved high vacuum producing unit that employs a dry vacuum pumping system to prevent pump oil-contamination of the gas sorption system and test sample being analyzed.
2. Description of Related Art
The use of vacuum volumetric sorption apparatus is well known in the prior art for measuring the surface area and the porosity of powdered and porous samples. While the overall systems may vary on certain features, all vacuum volumetric gas sorption systems have certain essential features which include primarily a high vacuum pump, at least one gas supply, sample container, a calibrated volume, and a pressure measuring means.
The high vacuum pumping system is critical to achieve sufficiently low pressures for accurate sample measurements. This is particularly important for microporous materials where low adsorption data is used to characterize pore size distribution, e.g. zeolites for gas separation and petroleum cracking (catalysts), as well as chemisorption where a few parts per million of residual pressure may contaminate active sites of industrially important catalysts. Of primary concern in present day systems however is to insure that the sample does not become contaminated.
Frequently, vacuum volumetric analyzers use a high vacuum turbomolecular drag pump for creating a vacuum in the sample vessel. The turbomolecular drag pump includes a high speed turbine that compresses the molecules toward the outlet side, reducing the pressure in the sample vessel compressing the molecules and forcing them out the exhaust outlet side of the turbomolecular drag pump. In order for this system to work effectively, a turbomolecular drag pump needs a roughing pump or foreline pump on its exhaust side to remove the compressed molecules being pushed out of the turbomolecular drag pump. Typically, the roughing pump is an oil lubricated rotating vane pump. In using the vacuum volumetric sorption apparatus, high vacuum requirements frequently reach 10.sup.-5 TORR.
One of the problems in using the high vacuum turbomolecular drag pump with the vacuum oil lubricated rotating vane pump as the roughing pump, is that the oil has a vapor pressure at room temperature. Therefore, it is possible that contaminants of oil vapor could have access to, and in fact, cause the sample that is being analyzed to be contaminated via backstreaming, thus defeating the entire purpose of the system and robbing it of its accuracy. Furthermore, pump oil may crack and decompose over time, producing additional volatile components.
One solution to prevent oil contamination in the sample has been to use, between the turbomolecular drag pump and the roughing pump, a canister that has material such as activated carbon or alumina that will trap oil vapor to prevent contamination of the sample. This canister is placed between the drag and rough pumps. This is called a foreline trap and has been used in vacuum volumetric analysis systems. One of the drawbacks to using this foreline trap is that the trapping material such as carbon has to be reactivated periodically, adding expense and downtime to the system and uncertainty as to when it is time to reactivate the filter material. Furthermore, the foreline trap limits the ultimate achievable vacuum and impedes the rate at which vacuum is achieved.
Another solution to eliminate pump oil contamination uses liquid nitrogen (or other suitable cryogen) somewhere along the vacuum line that would cause any oil contaminate in the vacuum line to freeze. This requires replenishing liquid nitrogen, which is costly and undesirable. If the trap runs out of the liquid nitrogen causing it to warm up, a large amount of oil may be released. Therefore, it requires vigilance to insure that the liquid nitrogen supply is constantly replenished to avoid the problem.
The present invention overcomes the problems encountered in present day systems by completely eliminating vacuum pump oil. The present invention provides an entirely dry pump system, by utilizing a diaphragm pump that is connected to and works in conjunction with the high pressure outlet side of the turbomolecular drag pump. Inasmuch as the turbomolecular drag pump uses such a small trace of lubricant (to prevent bearing wear), the system is essentially dry, the entire vacuum system remains dry, and sample contamination from oil is rendered impossible.
The present invention eliminates the need for a foreline trap and also eliminates the need for using a cold trap in an effort to trap oil contaminants as had been previously used and discussed above.